What is the normal magnitude of the bearing capacity for a raft foundation?

Raft Founadtion is normally provided if bearing capacity of soil is less say0.75 Ton / sq. ft or less.

In fact it depends on many factors that among them is the strength of soil beneath the foundation. This factors varies with the dimension of footing. The wider dimensions, the more magnitudes of bearing pressure. But, it should be considered that bigger dimensions create more weight which will reduce the amount of baring pressure. So, there must be a balance between foundation and soil resistance to acquire the best results.

Depends on :-

1-bearing capacity of soil

2- loads of columns

3-dimensions of columns

4- spacing between axes of columns 

You must get the soil investigation (geotechnical) report for the foundation to answer this question.

I support the answer of Mr. Younes.

There is no direct answer to this. If soil bearing capacity is sufficient then raft foundation could be provided, after all it is much cheaper than pile foundation.

there is no normal magnitude of B.C., it comes with the soil properties and types (sandy, granular..)

The allowable bearing capacity (qa) is the maximum bearing stress that can be applied to the foundation such that it is safe against instability due to shear failure and the maximum tolerable settlement is not exceeded. The allowable bearing capacity is normally calculated from the ultimate bearing capacity using a factor of safety (Fs).

less than 150kPa for clayey soils.